In the prior method of preparing one of sulfoxide derivative or the salt thereof, 2-[[4-(3-methoxypropoxy)-3-methylpyridine-2-yl]methylsulfinyl]-1H-benzimidazole sodium salt (general designation: Rabeprazole sodium salt) represented by the formula (I) wherein R1 represents a hydrogen atom, a methoxy group or a difluoromethoxy group; R2 represents a methyl group or a methoxy group; R3 represents a 3-methoxypropoxy group, a methoxy group or a 2,2,2-trifluoroethoxy group; R4 represents a hydrogen atom or a methyl group; respectively, and B represents a hydrogen atom, an alkaline metal or ½ alkaline earth metal, 2-[[4-(3-methoxypropoxy)-3-methylpyridine-2-yl]-methylsulfinyl]1H-benzimidazole (general designation: Rabeprazole) had been dissolved in aqueous sodium hydroxide solution and then freeze-dried to obtain the amorphous substance thereof (see Patent Publication 1), or an acetone complex of 2-[[4-(3-methoxypropoxy)-3-methylpyridine-2-yl]-methylsulfinyl]-1H-benzimidazole sodium salt had been freeze-dried to obtain the amorphous substance thereof (see Patent Publication 2).

The method of preparing an amorphous substance by freeze-dry is general means for obtaining drugs, as a solid, mainly unstable protein drugs and antibiotics.
However, such methods have serious problems in light of preparing time, cost or environmental protection, for example, 1) the production scale is restricted due to plant-capacity or -ability and a large scale plant for freeze-dry exclusive use is required in case of industrial scale production, and 2) additional drying step of removing water is required after freeze-dry in case of drugs which are unstable to water.
In addition, it has been, known to be difficult to make the particle diameter of the amorphous substance obtained by freeze-dry the same size. In particular, in case of compounds such as Rabeprazole, whose water-controlling is important due to their deliquescence, there was a problem, that is, it is difficult to add steps of grinding and screening.
[Patent Publication 1]
U.S. Pat. No. 5,045,552
[Patent Publication 2]
U.S. Pat. No. 6,180,652